1. Field of the Invention
The present invention relates, in general, to an intramedullary implant having an elongated body specifically designed so that the geometric stiffness thereof can be optimized.
2. Information Disclosure Statement
Stiffness of an implant is determined by both material and geometry. Materials of choice for implant devices are titanium alloy, cobalt-chrome alloy and 316L stainless steel with titanium alloy having one-half the stiffness of the later materials. Consequently, a designer is limited as to the effect of material on stem stiffness. The geometry of a device has a major impact on intramedullary implant stiffness. Hereafter, the effect of geometry on intramedullary implant stiffness shall be referenced to as geometric stiffness. Various intramedullary implants have previously been designed so that the flexibility or stiffness of the implant can be varied. A preliminary patentability search produced the following patents that appear to be relevant to the present invention:
Smith, U.S. Pat. No. 5,007,931, issued Apr. 16, 1991, discloses a porous coated prosthesis including an elongated stem member intended for cementless fixation in the medullary canal of a long bone. The stem member has one or more longitudinally extending channels. A porous medium for enabling and encouraging bone in growth is bonded to the bottom surface of each channel but is free of the sidewalls of the channels. The longitudinal channels are also disclosed as reducing the section modulus of the stem, to make the stem more flexible. The Smith patent teaches that the depth of the longitudinal channels "may be of any dimensions suitable for a particular implant."
Giacometti, U.S. Pat. No. 4,921,501, issued May 1, 1990, discloses a stem for a femoral prosthesis including a distal end having a cylindrical cross section. The resiliency of the stem increases distally due to a hollow formed in the distal end of the stem either by a series of stepped bores as shown in FIGS. 1 and 2c-2f, or by a constant diameter bore that extends angularly of the longitudinal axis of the stem as shown in FIGS. 3 and 4c-4f. The distally increasing resilience of the stem is due to the increasing decrease in the lateral wall thickness or total cross-sectional area.
Smith, U.S. Pat. No. 4,808,186, issued Feb. 28, 1989, discloses a femoral prosthesis having an elongated stem for insertion in the intramedullary canal of a femur. The stem has a longitudinal channel therein which lies generally in the coronal plane when the stem is implanted. The thickness of the stem laterally of the channel is variable between the proximal and distal ends to affect the moment of inertial at any given location along the length of the stem to thereby provide stem flexibility that substantially correlates to the flexibility of the bone.
Petrtyl et al., U.S. Pat. No. 4,743,263, issued May 10, 1988, discloses a hip prosthesis including a shaft for being implanted in a femur. The shaft is composed of at least two, spirally twisted elastic rods. The elastic rods have a "turn" larger than the diameter of the prepared cavity in the femur so that the elastic rods are stressed when inserted into the prepared cavity. The elastic rods may have a variable cross section.
Forte, U.S. Pat. No. 5,092,899, issued Mar. 3, 1992, discloses a prosthesis with an intramedullary stem that has flexibility comparable to that of the surrounding bone. A bore is disposed in the stem portion with the stem wall thickness uniform or varying from the proximal end to the distal end, depending upon the amount of flexibility wanted. The stiffness along the length of the stem may additionally be varied by varying the depth of the bore into the stem.
Farling, U.S. Pat. No. 4,997,444, issued Mar. 5, 1991, discloses a hip prosthesis having a stem constructed so that the modulus of elasticity thereof vary from one end to the other. The stem includes a plurality of alternating solid and mesh discs stacked one on top of the other with the relationship between the composite thickness of mesh and solid discs determining the modulus of elasticity at any region thereof.
Hofmann, U.S. Pat. 4,936,863, issued Jun. 26, 1990, discloses a hip prosthesis including a femoral component having an upwardly and laterally open slot formed in an upper region thereof to allow compression of the femoral component for easier installation into a resected femur. An antirotation fin can be inserted into the slot after the femoral component is implanted to prevent rotation of the femoral component and maintain the femoral component in an expanded state.
Pappas et al., U.S. Pat. No. 5,030,234, issued Jul. 9, 1991, discloses a femoral hip prosthesis including a stem and an extension for being connected to the distal end of the stem by a slip fit connection that reduces surface tensile forces in regions of the prosthesis adjacent the interface between the stem and the extension.
Tepic, PCT publication WO 89/01321, published Feb. 23, 1989, discloses a stem for a hip prosthesis having a central region provided with a regular pattern of anteroposterior cuts extending from the proximal regions of the stem to the distal end of the stem, rendering the stem stiffness adaptable to the stiffness of the receiving bone cavity.
Nothing in the known prior art discloses or suggests the present invention. More specifically, nothing in the known prior art discloses or suggests an intramedullary implant including an elongated body for being implanted in the medullary canal of a long bone, a first groove means for varying the geometric stiffness of the body, and a second groove for varying the geometric stiffness of the body, the first and second groove means having contours with varying cross-sectional shapes whereby the geometric stiffness of the body can be optimized.